Generally, exhaust gas flowing out through an exhaust manifold from an engine is introduced into a catalytic converter provided at an exhaust pipe and is purified therein. Then, noises of the exhaust gas are decreased while passing through a muffler and then the exhaust gas is discharged into the atmosphere through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas.
A denitrification (DeNOx) catalyst is used in one type of such a catalytic converter, and purifies nitrogen oxides (NOx) contained in the exhaust gas. When reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced by the DeNOx catalyst through an oxidation-reduction reaction with the reducing agents.
In order to decrease the nitrogen oxides, a three-way catalyst is used in a gasoline vehicle, and the three-way catalyst can simultaneously purify three harmful substances such as carbon monoxide, hydrocarbons, and nitrogen oxides by an oxidation-reduction reaction.
In the exhaust gas purifying apparatus such as the three-way catalyst, purification efficiency by a catalyst thereof is deteriorated when oxygen is intercalated in the catalyst under a fuel-cut condition as in coasting of a vehicle.
In order to solve this problem, when a fuel cut state is switched to a running state in which fuel is injected, fuel is injected so as to be richer than the stoichiometric air-fuel ratio, thereby purging the oxygen intercalated in the catalyst (this is referred to as catalyst purge). However, although the catalyst purge is performed, until the oxygen is purged, the purification efficiency of the catalyst is maintained in the deteriorated state, and thus, noxious exhaust gas may be discharged.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.